1. Field of the Invention
The present invention relates to an instrument for keratometry, and more specifically, to an improved keratometer that is capable of quantitatively measuring keratometric values for human eyes.
2. Description of the Prior Art
The prior art is aware of several types of medical instruments that have been developed to allow a medical examiner to obtain information about corneal curvature, surface regularities and dioptric refractive power of the cornea undergoing examination. A keratoscope is a type of medical instrument that gives qualitative information about corneal topography and curvature by allowing the medical examiner to observe the corneal reflection pattern produced by an object or objects of known geometric dimensions that are incorporated into and illuminated by the instrument. A keratometer differs from a keratoscope in that it provides quantitative measurement of corneal curvature, usually expressed as dioptric refractive power, as well as providing qualitative information about corneal topography.
In the prior art, keratometer devices have generally been derived from a single generic design, attributed to Helmholtz, that infers corneal power by measuring the image magnification of a known object geometry produced by corneal reflection utilizing the separation between two or more images of the object. Instruments of this type incorporate an illuminated mire image which is projected onto the cornea under examination. The reflected images are viewed by the examiner through an optical system aligned coaxially with the cornea. The optical system can incorporate a system of image doubling, through use of a bi-prism, risley prisms, or other optical construction, that causes formation of one or more secondary images displaced from the primary image. The amount of displacement can be adjusted mechanically or optically, and when brought into adjustment either by superimposition of mires, or tangency of mires, an indirect measurement of image size is inferred. The size of the reflected corneal image is proportional to corneal curvature and if the mire dimensions, optical system geometry, and image size are known, the curvature of the cornea can be quantitatively inferred.
Recently, keratometers have been used during eye surgery to allow the surgeon to monitor and eliminate any undesired astigmatism through modification of wound closure at the time of the surgery.
A "surgical" keratometer is a type of keratometer device that attaches to or modifies an operating microscope, allowing the surgeon to observe corneal topography and in some instances measure corneal curvature intraoperatively, on an eye undergoing surgery. This measurement is of value in that wound closure and suture tension can be modified, based upon the keratometric measurements, so as to minimize any undesired astigmatism induced by the operative procedure.
A surgical keratometer differs from a clinical keratometer in that it is designed to allow the observer (surgeon) to view the reflection of the keratometric mires through the surgical microscope optical system as he is viewing the surgical field. Furthermore, the eye under examination is anesthetised, through either general or local retrobulbar anesthesia of the patient.
The first device to be developed in this category was the Troutman Surgical Keratometer, which should actually be classified as a type of keratoscope (and not a type of keratometer) because it only provides for viewing of a number of light sources arranged in a circular pattern around the entrance pupil of the microscope, and is not capable of precise quantitative measurements of corneal curvature.
The Smirmaul Surgical Keratometer is a semiquantitative device, in that it does incorporate image-doubling of an object mire, and allows positional adjustment of the doubled image. When properly adjusted, inference can be made regarding corneal dioptric power. Because it cannot be accurately and reproducibly calibrated, however, its measurements of corneal dioptric power are prone to inaccuracy. It is somewhat useful in determining axis and power of corneal astigmatism, since this is a relative determination based upon the arithmetic difference between the values obtained for maximum and minimum curvature.
The Terry Keratometer is a surgical keratometer capable of quantitative measurement of corneal curvature. It incorporates an illuminated circular object mire placed immediately below the entrance pupil of the microscope, and a series of image-doubling prisms installed inside the housing of the microscope. The surgeon mechanically adjusts the prisms to merge two or more Purkinje images of the mire that appear to be located behind the corneal surface. This method indirectly measures the Purkinje image diameter by displacement of the doubled Purkinje images. Because it can be accurately calibrated, the instrument is capable of yielding reliable data. However, installing the Terry device requires mechanical alteration of the microscope, and electronic interfacing with the control circuitry for the microscope zoom function, both of which pose theoretical and practical objections to its design.
Finally, the Humphrey Auto Keratometer is a type of instrument that utilizes three rays of infrared light which are directly reflected off of the corneal surface to three separate photodetectors whose spatial geometry allows inference of corneal curvature in two orthogonal meridians. A chopper disc provides means for determining which infrared ray is received by each of the photodetectors.
These types of instrument design have several useful characteristics. First, the direct measurement of the size of a corneal reflected image in the past has been quite difficult, whereas the indirect method employed above was easy and optically simple. Second, the technique can be employed in the presence of microscopic oscillations and movements of the eye which are present in the awake individual, even while the subject is attempting steady fixation.
There is significant disadvantage to the use of image-doubling prisms in keratometer design, especially relevant to intraoperative use of a surgical keratometer. In this setting, the surgeon desires to view the operative field concurrent with the mires necessary to measure keratometry. With any image-doubling optical scheme, there will be doubling of not only the keratometry mires, but also of objects within the surgical field, creating a ghost image clearly visible to the surgeon. Also, because of the nature of image doubling prisms, some image-smear is usually created by the prisms which cause slight degradation of even the primary image. Both of these types of image degradations are undesirable to the surgeon.
The greatest disadvantage of the above keratometers is the fact that the surgeon must physically intervene to adjust the instruments while he is performing the surgery.
Surgeons are still seeking a precise keratometer to assist them during operative procedures while providing minimal interference with those same operative procedures.